Six groups of Macaca nemestrina were immunized with one of the following regimens (1) subunit gp160, (2) subunit gp120; (3) vaccinia-gp160 priming followed by subunit gp160 boosting, (4) vaccinia-gp120 priming followed by gp120 boosting, (5) vaccinia-gag/pol/env/ priming followed by pseudovirion boosting, and (6) parental vaccinia virus as controls. All animals were challenged intravenously with 20-200 macaque-infectious-doses of SHIVIIIB (SIV with env gene of HIV-1IIIB) 4 weeks after the last immunization. Complete protection was observed in all 6 animals in groups 3 and 4, but not in groups 1 and 2, indicating the importance of priming with recombinant virus. Protection was correlated with the presence of SHIV-neutralizing antibodies. Two animals in group 5 that had low levels of neutralizing antibodies were also protected, indicating that other mechanisms, including cross-reactive immune responses to core antigens, may also contribute to protection. During the last year, one of the control animals developed irreversible CD4+ depletion, indicating the pathogenic potential of SHIV III in pigtailed macaques. All animals are being held to obtain further information on the clinical course of infection. Immunized and protected animals are being held for rechallenge studies.